1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for information transmission in a radio communication system.
2. Related Art
In next generation multimedia radio communication systems, which have been actively studied in recent years, there is a demand for a system capable of processing and transmitting a variety of information (e.g., video and radio data) at a higher data rate in addition to the early-stage voice service. The radio communication system is designed for the purpose of providing reliable communication to a plurality of users irrespective of their locations and mobility. However, a wireless channel has an abnormal characteristic such as a fading phenomenon caused by a path loss, noise, and multipath, an inter-symbol interference (ISI), a Doppler effect caused by mobility of a user equipment (UE), etc. Various techniques have been developed to overcome the abnormal characteristic of the wireless channel and to increase reliability of radio communication.
A multiple input multiple output (MIMO) scheme is used as a technique for supporting a reliable high-speed data service. The MIMO scheme uses multiple transmit antennas and multiple receive antennas to improve data transmission/reception efficiency. Examples of the MIMO scheme include spatial multiplexing, transmit diversity, beamforming, etc. A MIMO channel matrix depending on the number of receive antennas and the number of transmit antennas can be decomposed into a plurality of independent channels. Each independent channel is referred to as a spatial layer or a stream. The number of streams is referred to as a rank.
There is an ongoing standardization effort for an international mobile telecommunication-advanced (IMT-A) system aiming at the support of an Internal protocol (IP)-based multimedia seamless service by using a high-speed data transfer rate of 1 gigabits per second (Gbps) in a downlink and 500 megabits per second (Mbps) in an uplink in the international telecommunication union (ITU) as a next generation (i.e., post 3rd 3 generation) mobile communication system. A 3rd generation partnership project (3GPP) is considering a 3GPP long term evolution-advanced (LTE-A) system as a candidate technique for the IMT-A system. It is expected that the LTE-A system is developed to further complete an LTE system while maintaining backward compatibility with the LTE system. This is because the support of compatibility between the LTE-A system and the LTE system facilitates user convenience. In addition, the compatibility between the two systems is also advantageous from the perspective of service providers since the existing equipment can be reused.
A typical radio communication system is a single-carrier system supporting one carrier. Since a data transfer rate is in proportion to a transmission bandwidth, the transmission bandwidth needs to increase to support a high-speed data transfer rate. However, except for some areas of the world, it is difficult to allocate frequencies of wide bandwidths. For the effective use of fragmented small bands, a spectrum aggregation (or bandwidth aggregation or a carrier aggregation) technique is being developed. The spectrum aggregation technique is a technique for obtaining the same effect as when a band of a logically wide bandwidth is used by aggregating a plurality of physically non-contiguous bands in a frequency domain. By using the spectrum aggregation technique, multiple carriers can be supported in the radio communication system. The radio communication system supporting the multiple carriers is referred to as a multiple carrier system. The carrier may also be referred to as other terms, such as, a radio frequency (RF), a component carrier, etc.
Meanwhile, a variety of uplink control information is transmitted through an uplink control channel. Examples of the uplink control information include an acknowledgement (ACK)/not-acknowledgement (NACK) used to perform hybrid automatic repeat request (HARQ), a channel quality indicator (CQI) for indicating a downlink channel state, a scheduling request (SR) for requesting radio resource allocation for uplink transmission, etc.
A plurality of user equipments (UEs) in a cell can transmit uplink information simultaneously to a base station (BS). The BS needs to be able to identify each UE's uplink information simultaneously transmitted. When the uplink information of each UE is transmitted by using a different frequency, the BS can identify the information. Frequency division multiplexing (FDM) is a multiplexing scheme in which a plurality of UEs are multiplexed by using different frequencies. However, the plurality of UEs in the cell may transmit the uplink information to the BS by using the same time-frequency resource. In order to identify each UE's uplink information transmitted using the same time-frequency resource, an orthogonal sequence may be used by each UE in uplink information transmission. Alternatively, a sequence having a low correlation may be used. As such, a multiplexing scheme in which the plurality of UEs are multiplexed by using different sequences is referred to as code division multiplexing (CDM). That is, uplink information of the plurality of UEs can be transmitted by being multiplexed using the CDM and/or the FDM. However, when the CDM-based information transmission method is combined with a multiple-input multiple-output (MIMO) technique, there may be a problem in that orthogonality is not maintained. When the orthogonality is not maintained, it becomes more difficult for the BS to identify information of each UE than a case where the orthogonality is maintained. As a result, reliability of radio communication may deteriorate, and overall system capability may become worse.
Accordingly, there is a need for a method and apparatus for effective information transmission by combining the MIMO scheme with the CDM and/or the FDM.